The present invention uncovers a two channel stereo display, which simultaneously generates a right and left image in discrete modulation channels, which differ by the polarization of their light beams. The invention relates to display and projection systems using micro electro mechanical systems (MEMS) as displays. More specifically, the invention relates to the chirality (handedness) of deflectable micro mirror devices (DMD) and uncovers solutions for some of the geometric problems associated with this handedness in stereoscopic systems.
The human vision does not perceive the polarization of light, nor can it discriminate states of polarization. Polarization does not influence the brightness nor the colors of an image. Therefore, in so called passive stereo display systems in the state of the art, polarization is used for coding left and right eye information in a combined beam. Goggles with “passive” polarizing sheets, decode (separate) these signals so that only light of one plane of polarization reaches the right eye, while light of the other plane of polarization reaches the left eye.
Image generation in digital display systems is achieved with spatial light modulators (SLM). These SLMs comprise a rectangular matrix of N×M electronically addressable discrete pixels. SLMs modulate the constantly bright incident light according to the control signal by removing light from the incident beam to generate darker parts of the image. The type of modulation classifies two groups of modulators: Liquid Crystal—SLMs, (LCD) and micro electro mechanical Systems (MEMS, e.g. DMD).
Liquid Crystal SLMs work either reflectively (e.g. LCoS) or transmissively (LCD). In either case, the modulators require prepolarized light of a certain linear polarization (e.g. “S”-polarized light). Each pixel is “marked” by rotating the plane of polarization (in the example given, the on-modulated beam is turned from “S” to “P”-polarization). “Off”-pixels differ in their polarization from “On”-pixels. In the case of transmissive LCD TFT displays “Off”-pixel light is absorbed (eliminated) by an analyzer. In the case of reflective LC-displays “On” and “Off”-pixel light is separated by a beam splitter downstream of the modulator which reflects light of one state of polarization while transmitting the other.
Two channel stereo projection systems with liquid crystal SLMs according to the state of the art use reflective LCoS SLM (liquid crystal on Silicon). LCoS systems modulate “On” and “Off” state only by modulating their plane of polarization, while the angle of incidence is the angle of reflectance (it is, usually, 0° which is normal to the SLM surface). As a consequence, both polarization states exist in either channel and have to be separated with some effort. For the discrimination of “On” and “Off”-beams additional beam splitters are necessary (U.S. Pat. No. 5,921,650, WO Pat. No. 03/065737). Otherwise channel separation and image contrast are insufficient (U.S. Pat. No. 6,547,396). Using classical MacNeille PBS a contamination of the “S”-channel with 5% “P”-light results in a contrast <=20:1 and a channel separation of less than 1:20. While image contrast in mono systems is generally used as a quality measure channel, separation in two channel systems has hardly been discussed, whereas in acoustic stereo systems this is an acknowledged specification.
MEMSs are SLMs modulating incident light by reflecting “On”-light in a different direction than “Off”-light. DMDs from Texas Instruments (e.g. U.S. Pat No. 5,600,383) achieve this by electronically controlled micro-mirrors which can be deflected in two stable positions, thereby reflecting incident light in two different directions (a “On” and a “Off”-direction).
As MEMSs do not depend on prepolarized light, and work independent of (plane or ratio of) polarization, (mono) MEMS display systems according to the state of the art work with unpolarized light. FIG. 1 shows schematically the operational principle of MEMS. Micro-mirrors (17) can be deflected in two positions (e.g. ±12° off the normal towards the MEMS surface in recent MEMS). If the incident light reaches the mirror under an angle of twice the mirror deflection (24°), it is reflected perpendicularly (18) from the MEMS surface as “On”-light (19) (“On”-position of the mirror). If the mirror is deflected to the alternate, the “Off”-position, the incident beam is reflected with an angle of 48° to the surface normal of the MEMS as “Off”-beam.
Stereo display systems with MEMSs have been described in the state of the art which are based on single channel operation (WO Pat. No. 02/32149A2, WO 02/076107A1). These projector devices generate in an alternating fashion (sequentially) left and right images or left and right image portions with a different polarization. If in these projector systems color is generated sequentially, e.g. using a color wheel (1-Chip DLP systems) each of three primary color components exists in two states of polarization (e.g. left- and right-handed circularly polarized light). 3 Chip DLP systems, which generate color simultaneously (parallel) still alternate right and left image display. In active systems this is achieved by synchronously switching of the light flow to one of the eyes (this is usually handled by an electronic shutter system in the goggles of an observer). In passive systems the polarization is switched by a polarization conversion system (located within or outside the body of the projection system) synchronously to the display of left or right image. Passive stereo goggles with corresponding analyzers alternatively feed right and left eye (e.g. Barco Galaxy Warp, Ref No R599655; www.vr.barco.com; Barco, Kortrijk, Belgium). A disadvantage of alternating polarization stereo displays is a halving of the maximum light flux to each eye and a halving of the maximum image display frequency.
Simultaneous stereo display systems with MEMS according to the state of the art are realized with two separate mono display systems which emit two light beams with different polarization, and which do not superimpose left and right image before the screen. While this approach evades the dynamical disadvantages of single chip MEMS stereo displays, it results in substantial ergonomical disadvantages (adjustment, inaccuracy of zoom and focus, differences in light flux) and in general leads to increased setup efforts. Dependent on the system used it can also lead to a halving of the light output.
As MEMS do not change the polarization state of light, our invention is based on polarization independent image generation with MEMS for 2 channel stereo displays. The present invention is characterized by simultaneous generation of both images for left and right eye and their superposition in a single display system. The presented 2 channel MEMS system feeds the corresponding image information simultaneously to both eyes (thus at least doubling the light flux to each eye) with a high image contrast (MEMS specific) and good channel separation. Our goal is to provide a system with a comprehensive dual channel architecture analogous to the high end stereo audio amplifier in one body. The present invention renders possible the 2 channel projection of images, which are free of flicker and color artefacts at a high image frequency. The invention ensures optimally adjusted superposition within the device and thus a view of high quality 3D images for right and left eye, which comes close to natural vision. This allows the observer to have a relaxed stereo view (in contrast to prior art solutions, where disturbances in image or projection quality may cause discomfort or even nausea); in addition a common beam and a single projection lens guarantee fast installation and ease of use.